1. Field of the Invention
The present invention relates to a mounting material, a semiconductor device using a mounting material, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
The method of manufacturing a semiconductor device comprises a mounting step for mounting a semiconductor chip to a die pad by using a mounting material and the subsequent wire bonding step. In the wire bonding step, it is desirable to use a gold wire. And it is desirable for the wire bonding temperature to set the range of about 250° C. to 300° C. in order to increase the connecting strength of the gold wire. Therefore, the mounting material is required to be resistant to temperatures higher than the temperatures noted above. It should be noted in this connection that, if the mounting material is melted under the temperatures under which the gold wire is used, the semiconductor chip is detached from the die pad, resulting in failure to achieve the wire bonding. Under the circumstances, a silver paste or a high temperature solder having a melting point of about 300° C. is used as the mounting material in the semiconductor device nowadays.
The silver paste is generally used in a semiconductor device in the form of a dispersion prepared by dispersing a silver powder in various resins or in the form of a paste or a film. However, the silver paste is insufficient in its heat conductivity. Therefore, a high temperature solder containing at least 90% of lead is used in place of the silver paste in a semiconductor device such as a power device severely requiring the heat dissipation properties.
However, in view of the earth environment problem, a Pb-free solder attracts attentions in recent years. Also, the application of a legal regulation to the manufacture and trade of electronic appliances using a lead series solder is being conducted, making it necessary to take appropriately countermeasures promptly.
Under the circumstances, it is necessary to put a Pb-free solder to a practical use in a semiconductor device. Proposed in, for example, Japanese Patent Disclosure (Kokai) No. 11-172354 and Japanese Patent Disclosure No. 11-207487 are materials prepared by adding In or Sn to a Zn—Al—Mg series alloy as a Pb-free high temperature solder material having a melting point of about 300° C. Also, a solder material prepared by adding In or Sn to a Zn—Al—Ge series alloy is studied in Japanese Patent Disclosure No. 11-172353.
However, in any of the solder alloy materials proposed in the prior arts exemplified above, a liquid portion appears in the vicinity of 200° C. or 145° C. Therefore, if the solder alloy material is heated to a high temperature of about 250° C., the solder material is re-melted so as to give rise to an inconvenience that the semiconductor chip is detached from the die pad. In addition, the Zn—Al—Mg series alloy has a high brittleness, giving rise to the problem that the peeling takes place in the thermal impact test.
As described above, the Pb-free high temperature solder material, which is known to the art, is not suitable for the practical use and leaves room for further improvement in terms of the brittleness. Such being the situation, a Pb-free solder material is scarcely used in the semiconductor device nowadays.